Wait, whilst his son was still alive and married to this wife of his?
Nope, he saw her half naked or something, wanted her, and then came down the revelation that treating your adopted kid as a biological member of the family is haram. To drive home this point (read: to get the girl) he married Zaynab after Zayd divorced her (read: after he was made to divorce her). Before Islam, the the Arabs believed it to be disrespectful to marry an adopted son`s ex-wife, as he was considered a blood relation, but yeah, you can see how such a belief would be detrimental to society

.